The majority of cancer death is attributed to metastatic disease, and the brain is one of the major sites of breast tumor metastasis. Even with the advanced treatment such as stereotactic radiosurgery (SRS), the median survival of breast cancer patients who developed brain metastases is less than a year. Regardless of high clinical significance, the pathological mechanism of brain metastasis is still poorly understood. We recently found that (i) c-Met signaling is able to induce the expression of a group of inflammatory genes which play critical roles in the generation of N2 neutrophils in the brain metastatic lesions, (ii) N2 neutrophils promote the self-renewal of cancer stem-like cells and suppress T cell mediated immune surveillance and (iii) the BBB permeable natural compound, PTER, suppresses brain metastasis by selectively targeting the c-Met. These novel and discoveries led us to hypothesize that (i) c-Met promotes brain metastasis and resistance to the radiation therapy by generating N2 neutrophils, and (ii) N2 neutrophils in turn enhance self-renewal of cancer stem-like cells and promote local immune suppression. We also hypothesize that pterostilbene (PTER), a BBB permeable natural compound suppresses brain metastasis and relapse after radio-therapy by suppressing c- Met signaling and thereby reducing the amount of N2 neutrophils. To test these hypotheses, we will (i) investigate how c-Met-induced neutrophil regulatory factors (NRFs) modulate neutrophil activities in the brain metastatic lesions (Aim1), (ii) decipher the mechanisms by which N2 neutrophils promote tumor progression and radio-resistance (Aim 2), and (iii) test the efficacy of PTER in suppressing BrM and post-SRS recurrence in vivo (Aim 3). The project is highly innovative. First, we have established a state-of-the-art system biology screening approach and found that overexpression of c-Met is strongly correlated with patients who developed brain metastasis and it is associated with an induction of NRFs which promotes phenotypic changes of neutrophils in metastatic sites. Therefore, the outcome of this project will reveal novel pathological mechanisms of how neutrophil modulation by cancer-secreted factors contributes to the brain metastasis. Secondly, the exact feed-back mechanism by which N2 neutrophils promote tumor progression and radio- resistance is poorly understood. We propose to decipher the pathway of reciprocal interaction between tumor cells, N2 neutrophils and T-cells in the brain microenvironment, which we believe will lead to a discovery of novel therapeutic approaches to target N2 neutrophils. Finally, we will examine the potential utility of the BBB- permeable natural compound, PTER, as a therapeutic agent and a radiosensitizer agent for treating brain metastasis. If our hypotheses are borne out, this work has the potential to be a novel paradigm that could significantly improve the treatment and prevention of breast cancer.